U.S. patent application Ser. No. 528,829, entitled "Rhenium Catalyst for Decreasing Nitrogen Compounds in Exhaust Gases", now abandoned, teaches that rhenium is useful in decreasing certain oxides of nitrogen found in exhaust gases of a motor vehicle. This application is assigned to the same assignee as the present application and is incorporated herein by reference.
The mentioned application discloses that rhenium can be used under reducing conditions to eliminate oxides of nitrogen found in an exhaust gas stream. In order to produce reducing conditions for the rhenium catalyst, the engine producing the gas stream is operated at a rich air fuel ratio between from about 12.9 to about 14.1 whereby the exhaust gas stream is deficient in oxygen and, therefore, reducing in nature. The exhaust gas stream passes over the rhenium catalyst in order to have oxides of nitrogen removed therefrom by catalytic action.
While rhenium is in its metallic form under reducing conditions, it will oxidize when exposed to oxygen. This can occur in an automotive vehicle when the air fuel ratio is lean, that is, when more oxygen is present than is necessary to burn the fuel present. Lean conditions of few seconds duration occur during various periods of normal engine operation.
WHEN OXIDIZING OR LEAN CONDITIONS OCCUR, RHENIUM OXIDIZES FROM ITS METALLIC FORM TO AN OXIDE HAVING A +4 VALENCE AND SUBSEQUENTLY TO AN OXIDE HAVING A +7 VALENCE. The +4 valence oxide of rhenium is stable and is not volatile at catalyst operating temperatures. However, an unprotected +7 valence oxide of rhenium has a sufficient vapor pressure to volatilize under catalyst operating temperatures. In order to protect rhenium from volatilization and thus maintain the operating efficiency of the catalytic converter for a prolonged period of time, the converter must be protected from oxygen. One way this can be accomplished is by a careful control of the air fuel ratio so that it always is on the rich side. Such a control on the air fuel ratio is both difficult and expensive.
As an alternate approach to attempting to control the air fuel ratio so that it is always on the rich side, we have developed a way of stabilizing the higher valence oxide of rhenium so that it is not volatilized away under converter operating conditions. It is, therefore, a principal object of this invention to provide a rhenium catalyst system which is effective in the reduction of oxides of nitrogen under reducing conditions but is not volatilized away when oxidized to its higher valence state when the catalytic converter is subjected to oxidizing conditions.